In recent years, for example, a display device such as a head-up display or a head-mounted display that projects a projection image onto a display arranged in front in a field of view of a user and displays the image in a state in which the image is superimposed on a background has been developed. A technology regarding such a display device includes, for example, a head-up display described in Patent Literature 1. In this head-up display of the related art, in order to prevent the visibility of the projection image displayed on the display from being obstructed by the brightness of environmental light, a sensor for measuring the environmental light is arranged near the display, and an output of a light source used for a display of the projection image (luminance of the projection image displayed on the display) is adjusted according to a result of the detection of the sensor.